1. Field of the Invention
The present invention relates to a lubricant for refrigerators and, more particularly, to a lubricant suitable for use in refrigerators in combination with a fluorocarbon-type refrigerant the molecules of which do not contain chlorine, such as R 134 a (1,1,1,2-tetrafluoroethane).
2. Description of the Related Art
Fluorocarbon-type refrigerants are known to the prior art and have been used in refrigerators as this type of refrigerants is chemically stable and exhibits little toxicity. It has been recognized in recent years, however, that chlorofluorocarbons such as chlorofluorocarbon R 12 (dichlorodifluoromethane), has been a cause of damage to the earth""s atmosphere such as breaking down the ozone layer in the stratosphere and of warming of the earth. For this reason, it has been agreed in the Montreal Protocol that the use of chlorofluorocarbon-type refrigerants is to be completely abolished after the year of 2000.
Under these circumstances, chlorine-free fluorocarbon-type refrigerants whose molecules do not contain chlorine, represented by fluorocarbon R 134a (1,1,1,2-tetrafluoroethane) have been proposed as substitutes for chlorofluorocarbon such as R-12. Compared with the chlorofluorocarbon R-12 which has been used conventionally, chlorine-free refrigerants such as fluorocarbon R-134a exhibit high polarity and poor compatibility (that is, poor mutual solubility or miscibility) with lubricants such as napthenic mineral oil or alkyl benzene which have been commonly used in refrigerators. In order to overcome this problem, various lubricants for refrigerators have been proposed such as polyalkylene glycol-type lubricants as disclosed in U.S. Pat. No. 4,755,316 and Japanese Patent Laid-Open No. 3-28296 and ester-type lubricants as disclosed in Japanese Patent Laid-Open No. 2-268068.
In chlorofluorocarbon-type refrigerants such as chlorofluorocarbon R-12, chlorine contained in the molecules contributes to the maintenance of extreme pressure performance in the refrigerant compressor. Unfortunately, however, the fluorocarbon R-134a and other chlorine-free fluorocarbon-type refrigerants cannot provide sufficient extreme-pressure performance because they lack chlorine, even when used in combination with the above-mentioned newly-developed refrigerator lubricants. Consequently, loss of energy and wear due to friction are increased in the sliding portions of the compressor such as bearings, pistons and seals, resulting in seizures in the worst case.
Known extreme-pressure additives used for refrigerator lubricants can be broadly sorted into two types; namely, sulfur-type additives and phosphor-type lubricants. A sulfur-type extreme-pressure additive is disclosed in Japanese Patent Laid-Open No. 57-8294. This additive forms a sulfide film on the sliding surface which imparts superior extreme-pressure performance so as to reduce friction in a refrigerator, but it is undesirably corrosive in nature and has impaired wear resistance.
Examples of phosphor-type extreme-pressure additives are alkyl-substituted phosphates disclosed in Japanese Patent Laid-Open No. 62-156198 and a combination of phosphite and tricresyl phosphate disclosed in Japanese Patent Laid-Open No. 62-156188. A compound which does not have a benzene ring or an alkylene group, e.g., trioleyl phosphate, exhibits only limited solubility to fluorocarbon-type refrigerant which do not contain chlorine in their molecules, e.g., fluorocarbon R-134a. The known phosphor-type extreme-pressure additives, therefore, not only fail to appreciably improve extreme pressure performance, but they also have the opposite effect of increasing wear of the sliding parts in the refrigerators. Tricresyl phosphate exhibits solubility to fluorocarbon-type refrigerants whose molecules do not contain chlorine, e.g., fluorocarbon R-134a. This additive, however, does not produce any remarkable effect on the improvement in the lubricating performance.
Under these circumstances, the inventors have developed and proposed, in Japanese Patent Laid-Open No. 2-216764, an alkylene-group containing phosphite and phosphate as an extreme-pressure additive for use in refrigerators which operate with chlorine-free fluorocarbon-type refrigerants such as fluorocarbon R-134a. This extreme-pressure additive is still unsatisfactory in that it does not provide sufficiently high extreme-pressure performance and in that it cannot be practically used.
Accordingly, an object of the present invention is to provide a lubricant for use in refrigerators which exhibits superior solubility to chlorine-free fluorocarbon-type refrigerant whose molecules do not contain any chlorine such as fluorocarbon R-134a, as well as superior anti-seizure and anti-wear characteristics.
Through an intense study on lubricants for use with chlorine-free fluorocarbon-type refrigerants in refrigerators, the inventors have discovered that the above-described object of the invention can be achieved by a lubricant having the following features.
Namely, according to the present invention, there is provided a lubricant for use in a refrigerator which operates in combination with (that is, is mutually soluble with, or in miscible with) a chlorine-free fluorocarbon-type refrigerant whose molecules do not contain chlorine, the lubricant containing a synthetic oil, and 0.1 to 10 wt % total based on the weight of the synthetic oil of a least one molybdenumoxysulfide compound. The molybdenumoxysulfide compound is selected from the group consisting of molybdenumoxysulfide diorganophosphorodithioate given by the following formula (1), molybdenumoxysulfide diorganophosphorodithioates given by the following formula (2), and mixtures thereof. Thus, compounds of formula (1) have the formula:
{(RO)2PS2}2Mo2SXOYxe2x80x83xe2x80x83(1) 
where R represents a hydrocarbon group having 3 to 20 carbon atoms inclusive, and X and Y are each a number which meets the conditions of 0xe2x89xa6Xxe2x89xa64, 0xe2x89xa6Yxe2x89xa64 and X+Y=4 and compounds of formula (2) have the formula:
(Rxe2x80x22NCS2)2Mo2SXxe2x80x2OYxe2x80x2xe2x80x83xe2x80x83(2) 
where Rxe2x80x2 is a hydrocarbon group having 3 to 20 carbon atoms inclusive and Xxe2x80x2 and Yxe2x80x2 are each a numbers which meets the conditions of 0xe2x89xa6Xxe2x80x2xe2x89xa64, 0xe2x89xa6Yxe2x80x2xe2x89xa64 and Xxe2x80x2+Yxe2x80x2=4.
General Description of the Invention
In a compound the the formula (1) used in the present invention, R may be any hydrocarbon group having 3 to 20 carbon atoms inclusive. Preferably, however, R is a primary alkyl group having 3 to 20 carbon atoms inclusive (more preferably primary alkyl group having 3 to 14 carbon atoms), a secondary alkyl group having 3 to 20 carbon atoms inclusive (more preferably secondary alkyl group having 3 to 14 carbon atoms) or an allyl group having 6 to 20 carbon atoms. Examples of such groups are: an isopropyl group, butyl group, isobutyl group, amyl group, 4-methyl-2-pentyl group, 2-ethylhexyl group, tridecyl group, lauryl group, oleyl group, linoleyl group, p-tertiary butyl phenyl group, nonylphenyl group and so forth. Each of these groups can be used alone or two or more of these groups may be used simultaneously in a single compound of formula (1). Among these groups, most preferably used are an isobutyl group, a 4-methyl-2-pentyl group, a 2-ethylhexyl group, and a tridecyl group.
When the number of carbon atoms of R exceed the above-specified range, the compatibility (that is, the vapor phase solubility or the liquid phase solubility or miscibility) of the lubricant with a refrigerant such as fluorocarbon R-134a is impaired. Conversely, when the number of the carbon atoms is below the above-specified range, the compatibility of a compound of formula (1) with the lubricating oil is impaired.
In a compound given by formula (1), X and Y are required to meet the conditions of 0xe2x89xa6Xxe2x89xa64, 0xe2x89xa6Yxe2x89xa64 and X+Y=4.
A compound given by formula (1) can be prepared by one of the processes which is disclosed in Japanese Patent Publication No. 57-24798, Japanese Patent Publication No. 57-24799, Japanese Patent Laid-Open No. 61-87690 and Japanese Patent Laid-Open No. 61-106587.
In a compound of formula (2) used in the present invention, Rxe2x80x2 may be any hydrocarbon group having 3 to 20 carbon atoms inclusive. Preferably, however, R is a primary alkyl group having 3 to 20 carbon atoms inclusive (more preferably primary alkyl group having 3 to 14 carbon atoms inclusive), a secondary alkyl group having 3 to 20 carbon atoms inclusive (more preferably secondary alkyl group having 3 to 14 carbon atoms inclusive) or an allyl group having 6 to 20 carbon atoms inclusive. Examples of such groups are: an isopropyl group, butyl group, isobutyl group, amyl group, 4-methyl-2-pentyl group, 2-ethylhexyl group, tridecyl group, lauryl group, oleyl group, linoleyl group, nonylphenyl group, p-tertiary butyl phenyl group and so forth. Each of these groups can be used alone or two or more of these groups may be used simultaneously in a single compound of formula (2). Among these groups, most preferably used are an isobutyl group, a 4-methyl-2-pentyl group, a 2-ethylhexyl group, and a tridecyl group.
When the number of the carbon atoms of Rxe2x80x2 exceed the above-specified range, the compatibility of the lubricant with a refrigerant such as fluorocarbon R-134a is impaired. Conversely, when the number of the carbon atoms is below the above-specified range, the compatibility of a compound of formula (2) with the lubricating oil is impaired.
In a compound given by formula (2), Xxe2x80x2 and Yxe2x80x2 are required to meet the conditions of 0xe2x89xa6Xxe2x80x2xe2x89xa64, 0xe2x89xa6Yxe2x80x2xe2x89xa64 and X+Y=4.
A compound given by formula (2) can be prepared by one of the processes disclosed in Japanese Patent Publication No. 53-31646, Japanese Patent Publication No. 55-40593, Japanese Patent Publication No. 56-12638, Japanese Patent Publication No. 57-24797, Japanese Patent Publication No. 58-50233 and Japanese Patent Laid-Open No. 62-81396.
There is no specific restriction in the synthetic oil used in the present invention, and any synthetic oil is usable provided that it exhibits preferably superior compatibility with the chlorine-free fluorocarbon-type refrigerant such as fluorocarbon 134a which does not contain chlorine in its molecules. Preferably, the synthetic oil is materially compatible with (that is soluble with) the chlorine-free fluorocarbon-type refrigerant such as fluorocarbon R-134a when the refrigerant is in a liquid state at a temperature of xe2x88x9230xc2x0 C. to 50xc2x0 C. and exhibits a kinematic viscosity of 2 to 50 cst at 100xc2x0 C. Preferably, the synthetic oil is chorine-free. Examples of a synthetic oil that may be suitably used include polyoxyalkylene glycol, modifications of polyoxyalkylene glycol, neopentylpolyol ester, dibasic acid ester, and fluorinated oil. Such an oil maybe used alone or two or more of such oils can be used simultaneously in the form of a mixture.
Practical examples of polyoxyalkylene glycol are: polyoxypropylene glycol, polyoxyethylene glycol and polyoxyethylene polyoxypropylene glycol, preferably having a molecular weight of 200 to 3000. The oxyethylene group and the oxypropylene group in the polyoxyethylene polyoxypropylene glycol may be random groups or block groups.
Examples of modifications of polyoxyalkylene glycol include: polyoxyalkylene glycol monoalkylether, polyoxyalkylene glycol dialkylether, polyoxyalkylene glycol monoester, polyoxyalkylene glycol diester, and alkylene oxide adduct of alkylene diamine. More practically, it is possible to use an ether of polyoxyalkylene glycol and a straight-chain or branched-chain alkyl group having 1 to 18 carbon atoms inclusive; an ester of polyoxyalkylene glycol and an aliphatic carboxylic acid having 2 to 18 carbon atoms inclusive; and a propyleneoxide adduct, an ethyleneoxide adduct, an ethyleneoxide propyleneoxide random adduct or ethyleneoxide propyleneoxide block adduct of ethylene diamine, diethylene triamine and triethylene tetramine. It is also possible to use, as the modification of oxyalkylene glycol, polyoxyalkylene glycol glycerol triether and a halide of, particularly chlorinated, polyoxyalkylene glycol.
The neopentyl polyol ester is preferably an ester of neopentyl polyol and an aliphatic carboxylic acid having 2 to 16 carbon atoms inclusive, preferably 2 to 9 carbon atoms inclusive, and more preferably an ester with trimethylol propane, pentaerythritol, dipentaerythritol and tripentaerythritol.
Examples of the dibasic ester suitably used is an ester of a bivalent carboxylic acid and a primary or secondary alcohol having 4 to 18 carbon atoms inclusive. Practical examples of such an ester are dibutylphthalate and dihexyladipate.
A perfluoroether as disclosed in Japanese Patent Laid-Open No. 3-7798 can be used as the fluorinated oil.
In the lubricant of the present invention for use in a refrigerator, molybdenumoxysulfide diorganophosphorodithioate given by formula (1) and/or molybdenumoxysulfide diorganodithiocarbamate given by formula (2) is contained in an amount which is 0.1 to 10 wt %, preferably 0.5 to 5 wt %, in total of the synthetic oil.
When both the molybdenumoxysulfide diorganophosphorodithioate given by formula (1) and molybdenumoxysulfide diorganodithiocarbamate given by formula (2) are simultaneously used, the mixing ratio between these compounds may be selected freely provided that the total content of these compounds fall within the range mentioned above.
When the content of molybdenumoxysulfide diorganophosphorodithioate of formula (1) and/or molybdenumoxysulfide diorganodithiocarbamate of formula (2) is below the range specified above, it is not possible to obtain satisfactory performance of the lubricant, whereas, when the upper limit of the above-specified range is exceeded, corrosion is undesirably promoted while the reduction in the friction loss is not so remarkable.
The lubricants for refrigerators of the present invention may, when desired and within the scope of the object of the invention, contain other extreme-pressure additives such as tricresyl phosphate, as well as an additive or additives ordinarily used in lubricants used for refrigerators employing flourocarbon-type refrigerants, such as a stabilizing additive, e.g., neopentyl glycol diglycidylether, polypropyleneglycol diglycidylether and phenyl glycidylether, and an anti-oxidation agent, e.g., xcex4-naphthylbenzylamine, phenothiadine and BHT. The content of such additive, when used, should be within a range which is ordinarily adopted in refrigerator lubricants.
The lubricants for refrigerator usage that are provided by the invention exhibit superior compatibility with chlorine-free fluorocarbon refrigerants having no chlorine in their molecules, such as fluorocarbon R-134a, as well as excellent extreme-pressure performance and anti-wear characteristics, thus offering distinguished lubricating performance when used together with such chlorine-free refrigerants in various refrigerators which operate with such type of refrigerants.
The weight ratio of fluorocarbon-type refrigerant to synthetic oil in a refrigerant composition of this invention can range widely; for example, from about 1:99 to about 99:1.